Super-Waterproof Cotton

31 October 2010
Posted by Chris Smith.

Waterproof clothing has got a lot more comfortable over the last 20 years with the invention of various breathable fabrics. One common strategy is to make the fabric super hydrophpbic or very very water repellant, this means that despite the fact that it is covered with holes so water vapour from sweat can escape, water Dew droplet, about 1mm in diameter, resting on a superhydrophobic leaf surfacedroplets can't get through because they are bigger than the holes. The bigger the holes the more breathable the fabric is, but to stay waterproof, you also need to make the fabric more hydrophobic. While there are superhydrophobic coatings which could make a sieve reasonably waterproof, the problem is that most of the hydrophobic coatings are not especially robust, and whilst your coat might be very water repelant when it is new, if you wash it, you also wash off the water repellancy.

Bo Deng and collegues at the Shanghai institute of applied physics have been working on this, they took a commercially available super-hydrophobic coating, a heavily fluorinated acrylate polymer. Instead of adding some kind of catalyst to cause the acrylate monomer to form a polymer they irradiated cotton soaked in a solution of the monomer with gamma rays.  Sometimes a gamma ray will hit the cotton, break some bonds in the cotton and allow the polymer to to start forming at this pont. This means that the polymer molecules are strongly bonded to the cotton.

The fabrics they produce will allow a droplet of water to roll around the surface without wetting it at all, and more impressively will keep this property after the equivalent of 250 washes.

As well as making great coats, they suggest that the cotton might be useful in floatation devices as if you made the filling of a lifejacket, or even a fisherman's coat or shirt, out of this superhydrophobic cotton, it would repel water, and so be covered with a layer of air and so produce significant floatation if you fell in the water.

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